Ammonium nitrate explosive package and three-stage method of initiation



Get. 28, 1969 N. E. BARR ET AL 3,474,729 AMMONIUM NITRATE EXPLOSIVE PACKAGE AND THREE-STAGE METHOD OF INITIATION Filed Feb. 21, 1968 INVENTOR5 Nan/7m 5. Ban Char/e5 5 7207/2, J/r

BY M1. 64;,

. I RN United States Patent US. Cl. 102-23 2 Claims ABSTRACT OF THE DISCLOSURE Greater useful energy and more efficient explosive decomposition of a dry explosive of the ammonium nitratehydrocarbon fuel type is obtained by a three-stage initiation method employing a small high explosive primary booster at one end of the main charge and a secondary booster of aluminized dry ammonium nitrate explosive located some distance away from the primary booster within the main charge. The initiation progresses through primary booster, about /s to /3 of the main charge and then through the secondary booster to generate the energy necessary for efiicient explosive decomposition of the remainder of the main charge.

SUMMARY OF THE INVENTION The requirements of blasting operations and of impulse generation for geophysical exploration bear little resemblance to military and demolition explosives requirements, so that blasting is a separate and distinct art. The object in blasting is to obtain the maximum amount of useful work for the least expense. This is quite different from the maximum destructive efiect usually sought in military explosives.

In an effort to reduce costs, blasting explosives have been based increasingly on ammonium nitrate during the last two decades. Ammonium nitrate is an attractive raw material because it is available in consistently high purity at fertilizer prices. However, when used alone, ammonium nitrate is not a very good explosive. Even when formulated with suitable fuels, auxiliary oxidizers and sensitizers, ammonium nitrate often fails to decompose completely during use, with considerable waste of the latent chemical energy of the charge. If large high explosive boosters are employed to obtain more efiicient de composition, often the resulting explosion has high initial peak pressure and shattering effect but fails to produce the useful work which is desired in blasting.

Briefly, we have discovered that the desirable weightlifting effect and efficiency of decomposition is obtained by means of a blasting method in which the following steps are employed:

(a) Detonating a small high explosive primary booster charge located near one end of an elongated charge of a dry ammonium nitrate-hydrocarbon fuel explosive so that explosion of the ammonium nitrate-hydrocarbon fuel charge is initiated,

(b) Continuing the explosion of the ammonium nitrate-hydrocarbon fuel charge until from about /5 to /3 of the charge is consumed and then (c) Detonating a relatively larger secondary booster charge of a dry aluminized ammonium nitrate-hydrocarbon fuel explosive which is located at a substantial distance from the primary booster charge and (d) Exploding the remainder of the main charge of ammonium nitrate-hydrocarbon fuel explosive.

The manner of operating the method is explained in detail, with illustrative examples, in the discussion which follows:

3,474,729 Patented Oct. 28, 1969 "ice The drawing is a cut-away illustration of a container which may be loaded with explosive for the purpose of practicing the invention, showing the primary and secondary booster charges in their proper positions.

DETAILED DESCRIPTION OF INVENTION The desired objective is the efiicient explosive generation of gas and maintenance of high pressure throughout the decomposition of ammonium nitrate, but with the avoidance of an initial peak pressure of short duration. A sufficiently high peak pressure is desirable for initiation and maintenance of the explosion of ammonium nitrate. However, if this high pressure is obtained as a sharp initial impulse there is much shattering and production of debris, rather than the moving of earth or rock in fragments of reasonable size.

In generation of an impulse in seismic geophysical exploration, a high initial pressure peak should also be avoided, if possible, maximum energy being desired in an impulse of long wavelength. In seismic charges which are exploded in water the desired type of explosion lifts a large column of water into the air. The following example illustrates the method of the invention in comparision with other methods of exploding a charge of the same explosive in an identical container.

Example A main charge of a dry explosive of the following composition was employed:

Percent by wt.

Ammonium nitrate (whole prills) (minimum) Ammonium nitrate (ground prills, minus 35 US. Standard mesh) (maximum) About 46.5 Diesel fuel 6 0.5

Variations of 1 to 2% in the proportions of whole prills and ground prills produced no significant differences in product performance. The product was colored with an oil-soluble dye which was employed to provide visible indication of efficiency of mixing.

All charges were placed in identical cylindrical cans, 8 inches in diameter and 25 inches long, (No. 2 in the drawing) having in the bottom end an axially located cylindrical recess or booster well inch in internal diameter and extending 4% inches into the interior of the can. In the booster well of each can was placed a commercial high explosive booster, inch in diameter and 4 /2 inches long, having in it a recess for placement of the detonating cap and containing about 17 to 20 grams of high explosive. Boosters obtained from different sources contained various common high explosives, primarily PETN, pentolite or RDX. All gave satisfactory performance. The weight of the entire charge in each explosive can was approximately 50 pounds. Four groups of test charges were prepared according to procedures A, B, C and D below.

(A) Dry explosive was loaded into the bottom of the can until the can was filled to a level about two inches above the tip of the booster well No. 2 in the drawing. Two pounds of the dry explosive were then mixed with two pounds of granular aluminum (EXXO -30) and the resulting four pounds of aluminized explosive was packed in a short length of polyethylene tubing 6 inches in diameter, closed at both ends with metal clips. The package of aluminized explosive (No. 3 in the drawing) was placed on top of the dry explosive in the can and then the remainder of the space (No. 4 in the drawing) was filled with more dry explosive and the can was closed. The size, formulation and location of the secondary booster in relation to the primary booster and main About 47 charge provided an explosive package in which the method of initiation operated in a preferred manner.

(B) The can was loaded only with 50 pounds of the dry explosive formulation so as to demonstrate the behavior of the explosive when explosion is initiated only by the primary booster.

(C) A mixture of 11.5 pounds of the dry explosive and 2 pounds of granular aluminum was poured in the can around the booster well and the remainder of the can was filled with the dry explosive and sealed. The charges prepared by this procedure were used to demonstrate the effect of merely increasing the energy of the explosive in the vicinity of the booster well by mixing in the same quantity of aluminum as was employed in the aluminized booster of procedure A.

(D) Dry explosive was mixed thoroughly with granular aluminum (EXXO 90-30) in the proportion of 85 pounds of dry explosive to 15 pounds of aluminum. A can was loaded with 50 pounds of this explosive so as to demonstrate the effect of increasing the energy of the entire charge instead of employing a secondary booster.

The charges were set off in water 210 feet deep, the impulse transmitted through the water being picked up by geophones and recorded with conventional seismic exploration equipment. The performance was judged both from the seismic record and by visual observation of the height of water column and color of any fumes given off. Results compared as follows.

Charge preparation procedure: Results These results illustrate the improved performance obtained by the method of the invention, even when compared to methods employed in charges prepared by procedures C and D which might be expected to give equivalent results. In procedure C the higher energy explosive was initiated first, before initiation of any of the main charge, employing the same amount of aluminum for energy increase. Apparently separation of the two boosters and initiation of a portion of the main charge before the detonation of the secondary booster is a necessary condition. In procedure D the energy of the entire charge was increased by addition of aluminum. This was the most expensive of all the charges prepared and was quite powerful but did not lift as large a column of water as the charge prepared according to procedure A.

In other experiments it was demonstrated that placing the container of secondary booster described in procedure A so that it touches the primary booster well gave inferior results. In another experiment a container was filled with an aluminum containing aqueous slurry blasting explosive and was substituted for the secondary booster of dry explosive employed in procedure A. As the slurry booster contained only 1 pound of material it was placed in contact with the primary booster well to make sure that the main charge and secondary booster were initiated simultaneously. The results were decidedly unsatisfactory, colored fumes indicating incomplete decomposition of the ammonium nitrate. On the basis of these and other experiments it has been concluded that a package prepared generally in accordance with procedure A is preferred for operation of the method of the invention. 4

The quality and particle size of the aluminum used in the explosive charges of this method are not particularly critical. It is wasteful and not very effective to use particles of metal which are larger than mesh size (U.S. Series). Preferably, cheap, readily available granular aluminum of less than 20 mesh size is used. The material designated as EXXO 90-30 in the above examples is about 91 percent aluminum, containing some aluminum oxide and some copper and is usually manufactured from industrial aluminum alloy scrap. Typical particle size distribution is approximately as follows (U.S. Screen Series).

Percent Bulk density is about 65 lb. per cu. ft.

This material is quite satisfactory for use in the aluminized booster charges employed in the method. Neither high purity nor extremely small particle size is necessary, as in some types of explosives. Use of paint grade aluminum flake is not recommended, as it is hazardous to handle and increases the cost of manufacture without substantial benefit to the performance of the explosive charge.

We claim:

1. In a blasting method, the steps:

(a) detonating a small high-explosive primary booster charge located near one end of an elongated charge of a dry ammonium nitrate-hydrocarbon fuel explosive so that explosion of the ammonium nitratehydrocarbon fuel charge is initiated,

(b) continuing the explosion of the ammonium nitrate-hydrocarbon fuel charge until from about to of the charge is consumed and then,

(c) detonating a relatively larger secondary booster charge of a dry aluminized ammonium nitratehydrocarbon fuel explosive which is located at a substantial distance from the primary booster charge and,

(d) exploding the remainder of the main charge of ammonium nitrate-hydrocarbon fuel explosive.

2. A blasting explosive package containing a main charge of dry ammonium nitrate-hydrocarbon fuel explosive in a rigid, elongated container having a recess at one end for locating a small high explosive booster charge and having located inside the main charge at a substantial distance from the small high explosive booster a relatively larger secondary booster charge consisting of aluminized dry ammonium nitrate-hydrocarbon fuel explosive.

References Cited UNITED STATES PATENTS 3,046,888 7/ 1962 Gordon l0222 3,082,689 3/ 1963 Grifiith et al. 10224 3,091,177 5/1963 Grebe l0222 3,342,132 9/1967 Partridge 10224 3,377,909 4/1968 Grant et al. 8620 VERLIN R. PENDEGRASS, Primary Examiner U.S. Cl. X.R. 102-24 

